Down syndrome, also known as trisomy 21, is a genetic disorder caused by an extra copy of chromosome 21. This extra genetic material leads to intellectual disability, characteristic facial features, and other health problems. Prenatal screening and diagnostic testing during pregnancy can detect Down syndrome before birth.
What is Down syndrome?
Down syndrome is a genetic condition in which a person has 47 chromosomes instead of the usual 46. It is caused by an extra copy of chromosome 21. This extra genetic material affects development, causing characteristic features and health problems.
Down syndrome is the most common chromosomal disorder, affecting approximately 1 in 700 babies born in the United States each year. The incidence of Down syndrome rises with increasing maternal age. Older women have a higher risk of having a baby with Down syndrome than younger women.
Characteristics of Down Syndrome
People with Down syndrome typically have certain physical features, health issues, and developmental challenges. Common physical signs and symptoms include:
- Flat facial profile
- Upward slanting eyes
- Small ears
- Short stature
- Poor muscle tone
Medical problems commonly associated with Down syndrome include:
- Heart defects
- Intestinal malformations
- Hearing loss
- Vision problems
- Thyroid dysfunction
- Dementia later in life
People with Down syndrome also face intellectual disability and developmental delays. The level of intellectual disability is usually mild to moderate. Developmental milestones like sitting, crawling, walking, and talking are delayed.
Causes of Down Syndrome
Down syndrome is caused by having three copies of chromosome 21 instead of the usual two copies. This extra genetic material is the result of an error in cell division during the development of the egg, sperm, or embryo.
There are three types of Down syndrome depending on how the extra genetic material is acquired:
- Trisomy 21: The most common cause (95% of cases). An egg or sperm cell contains an extra copy of chromosome 21, creating a total of 47 chromosomes at conception.
- Translocation: An extra part or whole extra copy of chromosome 21 becomes translocated (moved) to another chromosome (3-4% of cases).
- Mosaicism: An error in cell division shortly after conception leads to some cells having 46 chromosomes and some cells having 47 chromosomes (1-2% of cases).
Can Down syndrome be detected on ultrasound?
Sometimes Down syndrome can be detected through abnormalities seen on ultrasound, but ultrasound findings are not definitive for diagnosing Down syndrome. An ultrasound during pregnancy may raise suspicions about Down syndrome, leading to further confirmatory testing.
Soft Markers for Down Syndrome on Ultrasound
Certain ultrasound findings are considered “soft markers” for Down syndrome. Soft markers are abnormalities that show up more frequently in pregnancies affected by Down syndrome. However, they are not definitive diagnostic features of Down syndrome.
Common soft markers for Down syndrome that may be seen on ultrasound include:
- Increased nuchal fold thickness (fluid accumulation at the back of the neck)
- Echogenic intracardiac focus (bright spot in the heart)
- Short femur length
- Short humerus length
- Pyelectasis (enlarged kidney pelvis)
- Absence of the nasal bone
These soft markers may prompt doctors to recommend further testing to diagnose Down syndrome during pregnancy. However, they cannot definitively diagnose the condition on their own.
Structural Birth Defects Associated with Down Syndrome
Babies with Down syndrome have an increased chance of certain structural problems that may be visible on ultrasound. These include:
- Heart defects (up to 50% risk)
- Intestinal malformations
- Problems with the brain structure, spine, or limbs
If major structural problems are seen on ultrasound, doctors will likely recommend further testing such as amniocentesis to determine whether chromosomal abnormalities like Down syndrome are present.
Screening and diagnostic tests for Down syndrome during pregnancy
There are various screening and diagnostic tests available to detect Down syndrome during pregnancy. Typically a screening test is done first to assess risk, then a diagnostic test can confirm if Down syndrome is present.
Screening Tests
Screening tests estimate the chance that a pregnancy is affected by Down syndrome. They do not diagnose the condition definitively. Screening options include:
- First trimester screening: Includes a combination of ultrasound measurements and blood tests done in the first trimester, typically between 11-13 weeks.
- Second trimester screening: Blood tests measuring certain hormones and proteins in the mother’s blood, done between 15-20 weeks.
- Cell-free DNA screening: Screens for fetal DNA in the mother’s blood. Available from 10 weeks onward.
If screening tests show an increased risk for Down syndrome, diagnostic testing is recommended to confirm a diagnosis.
Diagnostic Tests
Diagnostic tests directly analyze the baby’s chromosomes to confirm or rule out Down syndrome. These tests carry a small risk of miscarriage. Diagnostic options include:
- Chorionic villus sampling (CVS): Cells from the placenta are removed through the cervix or abdomen and analyzed. Done between 10-13 weeks.
- Amniocentesis: Amniotic fluid is extracted through a needle inserted into the uterus and fetal cells are examined. Performed after 15 weeks.
- Percutaneous umbilical blood sampling (PUBS): Blood is taken from the umbilical cord and analyzed. Done after 18 weeks.
Diagnostic testing through CVS or amniocentesis can definitively diagnose Down syndrome and other chromosomal disorders during pregnancy by detecting extra chromosomes.
Conclusion
Down syndrome cannot be definitively diagnosed through ultrasound alone. While some soft markers and structural abnormalities may be seen on ultrasound in fetuses with Down syndrome, these do not confirm the diagnosis.
Screening tests like the combined first trimester screening assess risk for Down syndrome. If high risk, diagnostic tests like amniocentesis or CVS can then confirm Down syndrome by directly analyzing the fetal chromosomes. These tests carry a small risk of miscarriage.
Ultrasound is often used as part of the screening process for Down syndrome. It also helps detect any visible birth defects that may impact pregnancy management. But a normal ultrasound does not rule out Down syndrome, and ultrasound soft markers do not definitively diagnose it. Confirmatory diagnostic testing is needed to determine conclusively whether Down syndrome is present if screening tests indicate increased risk.
Frequently Asked Questions
Can you see Down syndrome on an ultrasound at 12 weeks?
At 12 weeks, sometimes early ultrasound markers may raise suspicion for Down syndrome. An enlarged nuchal fold (fluid at the back of the neck) and absent nasal bone may be seen. But these are not definitive for diagnosing Down syndrome. The combined first trimester screening done at 12 weeks assesses risk through ultrasound measurements plus a blood test. A high-risk result would prompt confirmatory diagnostic testing.
What week of pregnancy is Down syndrome detected?
Screening for Down syndrome starts early in pregnancy, but the condition is not definitively diagnosed until confirmatory diagnostic testing is done. Screening includes first trimester tests done between 11-13 weeks combining ultrasound and bloodwork. Diagnostic testing through CVS or amniocentesis confirms Down syndrome but is not typically done until the second trimester due to risk of miscarriage.
Can Down syndrome go away before birth?
No, Down syndrome cannot go away before birth. Down syndrome is caused by an extra chromosome 21 in the baby’s cells. This cannot resolve on its own or disappear during pregnancy. Down syndrome is a lifelong condition. However, false positive screening test results early in pregnancy could lead doctors to believe that Down syndrome is present initially, and then disappear if follow-up diagnostic testing is normal.
Can ultrasound detect Down syndrome at 20 weeks?
An ultrasound at 20 weeks cannot definitively diagnose Down syndrome. Structural problems like heart defects may be seen at this ultrasound that could raise suspicion and prompt testing. But some babies with Down syndrome have no structural issues evident on a 20 week ultrasound. Diagnostic testing like amniocentesis is needed to confirm Down syndrome.
How accurate is Down syndrome ultrasound?
Ultrasound alone cannot definitively diagnose Down syndrome. Ultrasound screening in the first trimester can detect markers like increased nuchal fold thickness. But these are not very sensitive or specific for Down syndrome. Upt to 20% of Down syndrome pregnancies show no abnormalities on this ultrasound. Confirmatory diagnostic testing gives a definitive diagnosis. Amniocentesis and CVS are over 99% accurate in detecting Down syndrome.
Key Takeaways
- Down syndrome cannot be definitively diagnosed on ultrasound alone.
- Screening tests like the combined first trimester screening assess risk for Down syndrome early in pregnancy.
- Soft ultrasound markers like increased nuchal fold thickness may raise suspicion but are not confirmatory.
- Diagnostic testing like amniocentesis or CVS is needed to confirm Down syndrome by chromosome analysis.
- Ultrasound is useful for detecting structural abnormalities that may be associated with Down syndrome.
- A normal ultrasound does not rule out Down syndrome.